It is often desirable to provide an accurate estimate of the amplitude and time delay responses of a network. The term network as used herein is intended to be used in a broad sense to include any entity to which input signals can be applied and which operates in some way on such signals to provide output signals therefrom. Thus, such term can be deemed to include circuits for processing, or otherwise handling signals, to include signal links, or signal paths, over or through which signals can be transmitted, and to include as well any combinations of one or more circuits and one or more signal links.
It is helpful to describe the invention by way of a specific example of a network for which the invention finds effective use. An example of one such network is a time division multiple access (TDMA) communications system. The measurement of the time delay and amplitude responses of such a TDMA system often arises, for example, in the context of, although not necessarily limited to, a TDMA satellite communication system. Such a system is normally made up of three major components, namely, a transmitter (sometimes referred to as an an uplink) earth station, a satellite, and a receiving (sometimes known as a downlink) earth station. The TDMA system to be measured is the concatenation of these three major components. Furthermore, the system must be capable of being measured in both directions, since the earth stations normally operate as full-duplex traffic stations, i.e., ones which simultaneously transmit and receive at different frequencies.
The time delay and amplitude characteristics are required to be measured over a specified bandwidth. For example, in TDMA satellites operated by the International Telecommunication Satellite Organization (INTELSAT) and described in the article by R. J. Colby, R. Parthasarathy, and D. W. Prouse, "An Introduction to Testing Techniques in the INTELSAT TDMA/DSI System," International Journal of Satellite Communications, Vol. 2, No. 13, July-September 1984, pp. 145-159, such bandwidth is 80 MHz.
The article suggests that a device making such a measurement be referred to as a "burst mode link analyzer" (BMLA), and recommends that it operate using a so-called "radar" approach to the measurement of a TDMA channel. In such radar approach, a signal generator at the transmitter earth station (referred to as the "BMLA" transmitter) generates a short burst of a probe signal in a designated time slot of a frame, as also described in more detail below. The probe signal, as described by Colby et al., comprises a short period pulse signal at the center frequency carrier of the band followed by short periods (bursts) of an offset pulse signal, the frequency of the offset signal being systematically varied in discrete steps from burst to burst or over a plurality of bursts in order to measure the entire TDMA system bandwidth.
In the radar approach, the BMLA compares the time of arrival (TOA) of the band center pulse with the TOA of the pulses at other frequencies, tallies their time differences, and determines the group delay thereof over the system bandwidth. Further, in the radar approach the amplitude is determined by detecting the level of the pulses at each frequency offset and comparing such levels to that of the band center pulse. The processed data are sent by the receiving station to the transmitting station over a service channel which is separate from the traffic channel.
A later article by P. Mahoney, "The INTELSAT TDMA Burst Mode Link Analyzer," International Journal of Satellite Communications, Vol. 3, Numbers 1 and 2, January-June 1985, pp. 171-177, describes generally a plan for building a BMLA for INTELSAT and presents a specific technique for implementing such a radar approach to amplitude and time delay measurements. Such paper describes the probe waveform as comprising three subbursts, namely, a reference subburst at the center frequency of the band, followed by two time-sequential subbursts at measurement frequencies symmetrically displaced with respect to the center frequency of the band. The relatively amplitude is measured by comparing the amplitudes of each of the three subbursts, and then dividing these measurements by the amplitude obtained at the center frequency of the band. The delay of each of the three subbursts is measured relative to a local standard. The time of arrival of the burst the burst at the band's center frequency is subtracted from the times of arrival associated with each of the other two subbursts to provide the group delay of the two other subbursts relative to the center frequency subburst.
While such articles generally describe a desirable goal to be obtained, namely, to obtain the amplitude and group time delay measurements as quickly as possible and in a manner which provides a relatively high degree of accuracy in such measurements, neither article discloses any specific system for achieving such goal. The technique of the invention described herein discloses particular approaches to achieving such goal using specific techniques for obtaining relatively fast and accurate measurements of the type desired.